FIG. 1 diagrammatically illustrates pertinent portions of a conventional communication receiver apparatus including an RF receiver 11 (embodied, for example, as an integrated circuit) coupled to a baseband processor 13 (embodied, for example, as a digital signal processor integrated circuit). The portions of the communication apparatus illustrated in FIG. 1 are cooperable for converting an analog IF signal 17 produced by the RF receiver 11 into a digital baseband signal 18 upon which a digital communication processing portion 16 performs desired digital communication processing operations. An A/D converter 12 in the RF receiver 11 converts the analog IF signal 17 into a digital signal 19. This digital IF signal 19 is input to a digital IF-to-BB converter 14 which converts the digital IF signal 19 into a digital baseband signal 10. The digital baseband signal 10 is then applied to a matched filter 15 which filters the signal 10 to produce the desired digital baseband signal 18.
One example of the digital IF-to-BB converter 14 is the so-called CORDIC (COordinate Rotation DIgital Computer) circuit which receives the digital IF signal 19 from the A/D converter 12 in sign-magnitude format, and multiplies this digital signal by digital sine and cosine functions. These operations translate the digital IF signal 19 into a digital baseband signal 10 that is split into its I (in-phase) and Q (quadrature) components which are then separately filtered by the matched filter 15.
An example of the matched filter 15 is a so-called “integrate and dump” filter, which essentially sums a prescribed number of individual samples, and then takes the average of that sum. This type of digital filter processing is also commonly known as decimation.
FIG. 2 illustrates a more detailed example of the prior art IF-to-BB conversion architecture of FIG. 1. The example of FIG. 2, in which the RF receiver 11 is a GPS (Global Positioning System) receiver, illustrates exemplary disadvantages associated with the architecture of FIG. 1. As shown in FIG. 2, the design of the digital IF-to-BB converter 14 (in this case a CORDIC circuit) and matched filter 15 in the baseband processor 13 can significantly limit the frequency planning options in the RF receiver 11. Due to the design of the CORDIC circuit 14 and matched filter 15 in FIG. 2, the frequency fC of the analog IF signal 17 must be (28/3)×fO, (where fO is the bandwidth of the received RF signal, for example 1.023 MHz), and the sampling rate fS used by the A/D converter 12 must be (112/3)×fO. The relationship between the IF frequency fC and the sampling rate fS is fS=4×fC, which is standard operation for many conventional CORDIC circuits.
The aforementioned requirements for the IF frequency fC and the sampling rate fS disadvantageously limit the frequency planning options in the RF receiver 11. In particular, the mixer circuitry (not explicitly shown) that produces the IF signal 17 from the input RF signal (not shown) is required to produce the IF signal 17 at fC=(28/3×fO), and the A/D converter 12 is constrained to sample the IF signal 17 at fS=(112/3)×fO. These frequencies fC and fs must have the aforementioned values in order to provide the digital baseband signal 18 at the sampling rate (fS=2×fO) expected by the digital communication processing portion 16. It should therefore be clear that the design of the CORDIC 14 and matched filter 15 significantly limits frequency planning options on the RF receiver 11.
Frequency planning flexibility can be important, because today's communications systems integrate more and more complexity into smaller and smaller spaces. In addition, more communication systems are integrated into single consumer appliances. For instance, early 3G mobile phones will include dual band GSM radios, a WCDMA radio, a Bluetooth radio and a GPS receiver. As a result, there are a plethora of signals that are generated within a single device at various frequencies. In addition these signals can interact with one another creating both wanted and unwanted signals at harmonic multiples of each signal. These signals can further interact with one another through device nonlinearities to produce new signals at either the sum or difference of any of these signals.
Consequently, the frequency planning of each radio must take into account all the other signals that can be present within a single device (as well as those signals that impinge upon the device's antenna). This is a complex task that requires judicious selection of each local oscillator (LO) and intermediate frequency (IF) signal source or information channel. By judiciously choosing these signal frequencies with respect to one another, the communication system designer can ensure these signal sources do not interact with one another in a fashion that degrades the performance of any of the individual radios within the device.
It is therefore desirable to provide for more flexibility in the frequency plan of the RF receiver in communication receivers of the type illustrated in FIGS. 1 and 2.
According to the invention, the digital IF-to-BB converter and the matched filter are integrated into the RF receiver, thereby advantageously avoiding the IF frequency and sampling frequency restrictions imposed by the baseband processor design in prior art architectures.